fs-native-extensions

Native file system extensions for advanced file operations

npm install fs-native-extensions

Usage

Write to a file using an exclusive lock:

const { open } = require('fs/promises')
const { waitForLock, unlock } = require('fs-native-extensions')

const file = await open('file.txt', 'a+')

await waitForLock(file.fd)

try {
  await file.write('hello world')
} finally {
  unlock(file.fd)
}

API

const granted = tryLock(fd[, offset[, length]][, options])

Request a lock on a file, returning true if the lock was granted or false if another file descriptor currently holds the lock.

To lock only a portion of the file, offset and length may be passed. A length of 0 will request a lock from offset to the end of the file.

On macOS, BSD locks are used and so only locks on the whole file are supported.

Note that the lock is only advisory and there is nothing stopping another process from accessing the file by simply ignoring the lock.

Options include:

{
  // If `true`, request a shared lock, i.e. a read lock, on the file. By
  // default, an exclusive lock, i.e. a write lock, is requested.
  // Be aware that an exclusive lock can only be granted to files that are
  // writable!
  shared: false
}

await waitForLock(fd[, offset[, length]][, options])

Request a lock on a file, resolving when the lock is granted. If another file descriptor holds the lock, the lock will not be granted until the other file descriptor releases the lock.

Options are the same as tryLock().

const granted = tryDowngradeLock(fd[, offset[, length]])

Request a downgrade from an already held exclusive lock to a shared lock, returning true if the lock was granted or false if another process currently holds the lock. If false is returned, the exclusive lock is lost and must be requested again.

On Windows, the downgrade will happen atomically and be immediately granted.

await waitForDowngradeLock(fd[, offset[, length]])

Request a downgrade from an already held exclusive lock to a shared lock.

On Windows, the downgrade will happen atomically and be immediately granted.

const granted = tryUpgradeLock(fd[, offset[, length]])

Request an upgrade from an already held shared lock to an exclusive lock, returning true if the lock was granted or false if another process currently holds the lock. If false is returned, the shared lock is lost and must be requested again.

await waitForUpgradeLock(fd[, offset[, length]])

Request an upgrade from an already held shared lock to an exclusive lock.

unlock(fd[, offset[, length]])

Release a lock on a file.

await trim(fd, offset, length)

Create a hole in a file at offset for length bytes. On file systems that support sparse files, file blocks wholly covered by a hole will take up no physical space.

On Windows, the file must first be marked sparse using sparse(fd). Otherwise, zeros will be explicitly written to the hole.

await sparse(fd)

Mark a file as sparse. On Windows, this operation is required before holes can be created in the file. On other systems, this operation has no effect.

await swap(from, to)

Swap the paths from and to, making from assume the identity of to and to assume the identity of from.

On Windows, the swap is performed by first moving to to a temporary path, then moving from to to, and finally moving the temporary path to from.

On macOS and Linux, the swap is performed atomically.

License

Apache-2.0